High strength, high ductility steel sheets can be used for internal and exterior body panels, chassis parts, etc., for reducing the weight of automobiles. Hot-dip galvannealed steel sheet may be used in such applications to provide good corrosion resistance. However, C and Mn which may be added to steel (e.g., steel sheet) to increase strength can retard alloying during galvanization. Obtaining both strength and a sufficient degree of alloying in such steel sheet can be difficult. For example, a steel sheet containing 0.2% or more Si can exhibit insufficient wettability of the plating, and alloying may also not proceed easily, when applying a conventional Senzimir type hot-dip galvanizing technique to such steel sheet.
A method for producing hot-dip galvannealed steel sheet which utilizes Ni preplating of a base steel sheet containing 0.2% to 0.5% Si, which may help to address these shortcomings, is described, e.g., in Japanese Patent No. 2526320, the entire disclosure of which is incorporated herein by reference.
For example, a presence of P in steel can inhibit and delay an alloying reaction of zinc. An alloying time longer than that used for ordinary steel sheet may be required, which can reduce productivity. Further, when using a single manufacturing line to produce both steel sheet with a fast alloying speed (for example, ultralow carbon steel sheet to which Ti or Nb is added) and steel sheet to which P is added, it may be necessary to optimally manage an Al concentration in the hot-dip galvanizing bath, alloying treatment conditions, etc., and thus processing of such materials may become complicated.
In view of the problems described herein, it may be desirable to improve alloying speed of P-containing steel sheet. For example, a processing technique which includes preplating high tension steel sheet containing P with Ni, heating it under predetermined conditions, hot-dip galvanizing it, then heat alloying it under predetermined conditions is described, e.g., in Japanese Patent No. 2526320, the entire disclosure of which is incorporated herein by reference. However, it may still be difficult to produce high strength, high ductility hot-dip galvannealed steel sheet exhibiting a strength greater than 590 MPa using the technique described in this Japanese patent. A long soaking time may also be preferred to secure a sufficient degree of alloying when using such technique. As a result, both strength and ductility can drop significantly, so application of this technique to automobile internal and exterior body panels, chassis parts, etc. having a complicated shape may be limited. Further, when applying the technique described in this Japanese patent to a base steel sheet containing P, a plating appearance may likely become uneven, which may thus be unsuitable for automobile applications, e.g., for sheet used as exterior panels.
A further processing technique which includes annealing P-containing steel sheet, pickling such steel sheet, cleaning the surface, then galvanizing it, plating it, and heat alloying it is described, e.g., in Japanese Patent Publication (B2) No. 7-9055, the entire disclosure of which is incorporated herein by reference. A heat alloying treatment time used in this technique can be relatively long, and obtaining an appearance of sufficient quality for application to automobiles, in particular for external panels, may be difficult.